In order to effect autofocusing for excellent followability with respect to a moving photographic object, a well-known autofocus, single lens reflex camera comprises:
a focus detector for detecting a deviation of a photographic lens from an in-focus position with respect to a photographic object;
a mirror driver for moving a mirror for reflecting a light beam from the photographic object to a viewfinder optical system, into and out of an image-forming optical path extending from the photographic lens to a film;
a lens driver for moving the photographic lens to the in-focus position in accordance with the deviation detected by the focus detector, the lens driver having a function for moving the photographic lens during retraction of the mirror by the mirror driver; and
an aperture controller for stopping down the light beam from the photographic object, the aperture controller having a function for aperture adjustment during retraction of the mirror by the mirror driver.
In this type of camera, the aperture adjustment by the aperture controller and the lens movement by the lens driver are carried out simultaneously during retraction of the mirror by the mirror driver.
The aperture controller includes an encoder for detecting the number of rotations of a motor for driving aperture blades, while the lens driver includes an encoder for detecting the number of rotations of a motor for driving the lens. The numbers of pulses output from the two encoders are counted for controlling the aperture adjustment by the aperture controller and the lens movement by the lens driver to be made by predetermined amounts, respectively.
With the structure of the above, it is impossible, because of a limitation to program processing time, to input the pulses from the two encoders to a microcomputer which controls a photo-taking operation of the camera, and count the numbers of pulses simultaneously. Therefore, a high-speed, high-performance, i.e. costly microcomputer should be used to perform such simultaneous count. Either number of pulses could be miscounted which impairs a precise aperture adjustment or movement of the photographic lens to an in-focus position. Thus, conventionally, the load of the microcomputer is mitigated by providing a separate counter circuit for counting the number of pulses output from the encoder included in the aperture controller, and outputting a counting completion signal to the microcomputer after a predetermined number of pulses has been counted. The pulses from the encoder of the lens driver are directly input to the microcomputer for counting its number.
However, in the conventional camera noted above, the microcomputer may not be able to count the pulses output from the encoder of the lens driver, because of the limitation to the program processing time, while an interrupt process for suspending the aperture adjusting operation by the aperture controller is prosecuted as the counting completion signal is input from the counter circuit. This results in an error in the lens movement for the number of rotations of the lens driving motor corresponding to the number of pulses input during such period. Consequently, the camera has the disadvantage of providing ill-focused photographs although it has an autofocusing function with good followability to a moving photographic object.
While the need for the separate counter circuit is a disadvantage, it is difficult for the counter circuit to absorb chattering which would be easy where a program of the microcomputer is used. This requires an additional, chattering absorption circuit or an optical system composed of expensive parts such as a pulse disk and a photo-interrupter instead of a mechanical contact type encoder included in the aperture controller.